topic 28127

# Allowable current density for zinc plating?

A discussion started in 2004 but continuing through 2020

2004Q. Amps per square Ft? We have metal product electroplated after forming. My question is in regards to zinc plating. Is there a target amps per square ft. that is required to obtain a nominal coating thickness of .0005? If so should I be able to calculate surface area by the particular parts we mfg. and determine an appropriate lot size to obtain the nominal thickness required? thanks for any input.

Joe Comptonaccess hardware - Honeoye Falls, New York

2004

A. If you look in the plating process supplier's technical data sheet or a plating book like the Metal Finishing Guidebook you will sort of find your answer, but you won't like it because it will probably say something like 4-60/ASF, and the range will be too broad to help you much. About 20 ASF is probably typical, but even that won't quite answer your question, and might mislead you.

The real way to look at it is that no plating shop is going to plate slower than they have to, so what controls the ASF is not what ultimate plating thickness they are shooting for, but what ASF they can successfully plate at. So, what you are looking for is whether 20 ASF, or somewhat less or somewhat more, is the ASF they must limit themselves to. Then you can divide the available amperage of the rectifier by that number, and that's how many square feet you can plate in one load. Also, you can then assume some efficiency (which might be 80 percent for acid zinc, 50 percent for cyanide zinc, and somewhere in between for alkaline non-cyanide) and then apply Faraday's Law to see how long it will take to get .0005" at 20 (or whatever) ASF. The Metal Finishing Guidebook has a handy chart in the back which combines Faraday's Law with the various conversion factors and tells you how many amp-hours per square foot to get .001" thickness at 100 percent efficiency.

For zinc it's 14.3, so you need 7.15 for .0005". Say 80 percent efficiency, then you need 8.94 amp-hours per square foot. If you can plate at 20 ASF, that's a plating time of 27 minutes. I don't think that's terribly far off, but hopefully a practicing zinc plater will correct me.

Ted Mooney, P.E.

finishing.com - Pine Beach, New Jersey

Striving to live Aloha

2004

A. Dead on target Mr Mooney. We generally decide upon ASF depending on the part configuration. The part that has obtuse angles sticking out are better suited to lesser currents as low as 10 ASF and longer plating times, and dead flat parts can be plated with currents as high as 40 asf with reasonably uniform plating thickness.

January 17, 2020

Q. What is the upper voltage limit for NAZ NaOH baths (rack plate)? Chemical data sheets and CEF literature both state a max of 6 volts. But I cannot seem to find much information about the harm or risks for running higher voltage?

I am trying to find all ways to maximize our bath's through-put. We use steel anodes with a separate zinc regeneration tank. Curiously, we already operate in the 6-7 volt range, our main loads are 120-150 sq ft, and at this voltage we achieve 8-10 ASF. Thickness requirement for these loads is .0003" minimum. Due to mechanical issues (hoists), our current plating time is 45 minutes. This same line used to run at a 36-minute plating rate. We are in the process of upgrading our hoists, I am just preparing for the plating bath to be the new "choke-point".

Metal finishing - Lincoln, Nebraska USA

January 2020

A. Hi Bruce. What does 'NAZ' mean? Obviously, the ideal situation is when the design is such that everything "chokes" simultaneously -- i.e., you have the appropriate number of plating cells and the appropriate hoist capacity, that you still have enough cleaning time & acid dip time, sufficient drain time, a non-wasteful time-motion diagram, enough load/unload time, etc. How many plating cells do you have, and is the time-motion diagram such that all are full, or is one always empty ('put & take' programming)?

I am not familiar with voltage limits for plating processes; I've personally never heard of it. Rather, there are ASF limits beyond which the plating burns, based on, as our knowledgable colleague Kozem notes, the shape of the parts and other factors. Electroplating is definitely a science, but unfortunately that doesn't mean that our knowledge is sufficient that every answer can be mathematically derived from first principles. You currently plate at 8 to 10 ASF for 45 minutes and you used to successfully plate for only 36 minutes, so you must have been plating at:

45/36 X (8 to 10), or 10 to 12.5 ASF

Platers plate at as high a current density as they can without burning. I would guess, but it's only a guess, that you can probably plate at a higher current density than 10 to 12.5 ASF ... but, as Kozem says current density might be limited to 10 ASF or might be as high as 40 ASF depending on part shape, and I can't challenge those numbers.

Regards,

Ted Mooney, P.E. RET

finishing.com - Pine Beach, New Jersey

Aloha -- an idea worth spreading

January 21, 2020

Q. My apologies, NAZ (Non-cyanide Alkaline Zinc). I was taught years ago that gains in plating speed drop off above 6-7 volts in an alkaline bath, wasting energy, and potentially slowing the plating speed. Is this incorrect? I've posed this same question to our chemical supplier, they are researching this as well. We have three plating cells, all of which remain full the entire cycle. We just checked sodium carbonates, they are at 45 g/L, which surprises me since we have never treated for carbonates. I appreciate the feedback!

Bruce Brady [returning]Metal finishing - Lincoln, Nebraska USA

January 2020

A. Hi again. We agree on the fact that attempting to plate at beyond some certain empirically determined plating speed doesn't work properly because it generates too much hydrogen and/or burns the plating. I personally have just never heard of this plating speed issue expressed in terms of voltage rather than current density, so I can't comment -- although others hopefully will.

Working from the "Electrochemical Equivalents" chart in the MFG, it takes 14.3 ASF for 60 minutes to deposit .001" of zinc at 100% efficiency. Your efficiency should be 75%, maybe a bit higher, so 14.3/.75 = 19 ASF for 60 minutes to deposit .001" or 5.7 ASF for 60 minutes to deposit 0.0003". So at 10 ASF it will take about 34 minutes.

With 3 plating cells at 5 loads per hour you'd get about 35 minutes of plating time. If, with the new hoist, you can plate 7 loads per hour, that would be about 24 minutes of plating time and would require successful plating at 14 ASF. That assumes plating thickness is uniform, which it never is, so you'll actually need a bit more to reach a minimum thickness. Without seeing the parts, let alone actual experience with them, I can only guess, but I think it will work :-)

Of course, 7 loads per hour is not a lot, but you'd probably need additional plating cells to get beyond it.

Regards,

Ted Mooney, P.E. RET

finishing.com - Pine Beach, New Jersey

Aloha -- an idea worth spreading